I am finishing oak trim (casing, baseboards, and quarter round). I am using Minwax fast drying polyurethane clear satin. The instructions call for 2 or 3 coats. If I ensure that the 1st and 2nd coat adequately cover the trim, is the 3rd coat necessary or advised?
It is not necessary. Adding another coat of poly would make the finished project a little tougher and resistant to wear, but it also shines it up a bit more, which may or may not be an effect you are wanting. In the case of baseboards and trim, they aren't really going to see much wear anyway, so another coat isn't giving you much benefit.
So as long as you're happy with the results, there is no need to add another coat. The instructions for finishes are notorious for directing the user to apply more than is strictly necessary.